You don't smoke. Check. No one in your family smokes. Check. You're in the clear from the hazards of tobacco smoke, right? Not so fast.

You don't smoke. Check. No one in your family smokes. Check. You're in the clear from the hazards of tobacco smoke, right? Not so fast. Now we have to worry about third-hand smoke.

Never heard of it? Neither had I until I came across a study showing harmful effects in mice exposed to the odor and residue of tobacco smoke that lives on long after active smoking has stopped. The stuff can stick to furniture and other surfaces and become embedded in carpeting, drapes, clothes, and just about anything else that gets in its way.

The really scary thing is tobacco smoke becomes progressively more toxic as it goes from first-hand (active smoking) to second-hand to third-hand smoke, according to research cited by authors of the mouse study, published online in PLOS One.

"The first complete ban in the world on indoor smoking in all public spaces (including bars and restaurants) occurred in 1990 in San Luis Obispo, Calif.," Manuela Martins-Green, PhD, of the University of California Riverside, and co-authors said in their introduction to their study.

"That legislation, and its expansion to many countries, was achieved only because of clear scientific evidence that second-hand smoke is dangerous to nonsmokers. Now, more than 20 years later, evidence is emerging that third-hand smoke (THS) exposure potentially poses similar health risks, especially for children. Several studies have affirmed THS as an underappreciated public health hazard."

In an effort to further the understanding of third-hand smoke, Martins-Green and colleagues studied mice raised in an environment that included materials previously exposed to second-hand smoke. The animals can move about freely, thereby mimicking conditions of third-hand smoke exposure in children of smokers.

Investigators examined the effects of exposure to third-hand smoke on the mice's liver, lungs, skin, and behavior and compared the findings with those from a control group of mice not exposed to third-hand smoke.

The smoke-exposed animals excreted elevated levels of the tobacco-specific carcinogen NNAI, comparable to levels observed in children exposed to second-hand smoke, the authors reported.

As compared with the control mice, the smoke-exposed mice had increased lipid levels, evidence of non-alcoholic fatty liver disease, excess collagen and cytokine production in lung tissue, and evidence of impaired tissue-healing in skin (similar to slow-healing incision areas in smokers who undergo surgery). Moreover, mice exposed to third-hand smoke exhibited hyperactivity.

Combined with existing evidence, the results provide "a basis for studies on the toxic effects of third-hand smoke in humans and inform potential regulatory policies to prevent involuntary exposure to third-hand smoke," the authors concluded.

Should we be scared? Probably not, at least not yet.

A spokesperson for the American Cancer Society referred me to the ACS website, which has regularly updated information on all sorts of cancer research, including third-hand smoke. So far, this is what the ACS knows:

"There's no research in the medical literature as yet that shows cigarette odors cause cancer in people. Research does show that particles from second-hand tobacco smoke can settle into dust and onto surfaces and remain there long after the smoke is gone -- some studies suggest the particles can last for months. Researchers call this third-hand smoke or residual tobacco smoke."

The ACS agrees with the authors' assessment that evidence on third-hand smoke is accumulating. Studies of smokers' homes have found tobacco-specific carcinogens in household dust. Other studies have shown that exposure to third-hand smoke can alter DNA in cells and that residual particles from tobacco smoke can form substances that have even greater cancer-causing potential.

"No actual cancer risk has been measured at this time, but the health risks of third-hand smoke are an active area of research," according to the ACS.

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